"Who turned off the lights?"

Solution 1:

So, you are asking why we say "who turned off the lights?" instead of:

*Who did turn off the lights?

Or something along those lines.

The difference is in whether the WH-word (e.g. who/what/when/where/why/how) is the subject or not. If it is the subject, then you just put the Wh-word where you would have put the subject, and nothing else about the sentence changes. But, in any other case, we get the verb movement and possible introduction of "do/does" into the sentence that you are expecting:

What did you turn off?

In this case, "what" is the object of the sentence.

When did you turn off the lights?

In this case, "when" is an adverb, again not the subject of the sentence.

If you want to know the real deep answer to why it patterns this way, there is an area of syntax called WH-movement that deals with these kinds of things.